[PATCH RFC] fs: reduce pointer chasing in is_mgtime() test

[Jeff Layton <jlayton@kernel.org>]

The is_mgtime test checks whether the FS_MGTIME flag is set in the
fstype. To get there from the inode though, we have to dereference 3
pointers.

Add a new IOP_MGTIME flag, and have inode_init_always() set that flag
when the fstype flag is set. Then, make is_mgtime test for IOP_MGTIME
instead.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
I had always had a little concern around the amount of pointer chasing
in this helper. Given the discussion around Josef's fsnotify patches, I
figured I'd draft up a patch to cut that down.

Sending this as an RFC since we're getting close to the end of the merge
window and I haven't done any performance testing with this.  I think
it's a reasonable thing to consider doing though, given how hot the
write() codepaths can be.
---
 fs/inode.c         | 2 ++
 include/linux/fs.h | 3 ++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index 838be0b49a63bd8d5700db0e6103c47e251793c3..70a2f8c717e063752a0b87c6eb27cde7a18d6879 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -243,6 +243,8 @@ int inode_init_always_gfp(struct super_block *sb, struct inode *inode, gfp_t gfp
 	inode->i_opflags = 0;
 	if (sb->s_xattr)
 		inode->i_opflags |= IOP_XATTR;
+	if (sb->s_type->fs_flags & FS_MGTIME)
+		inode->i_opflags |= IOP_MGTIME;
 	i_uid_write(inode, 0);
 	i_gid_write(inode, 0);
 	atomic_set(&inode->i_writecount, 0);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index aa37083436096df9969d2f63f6ec4d1dc8b260d2..d32c6f6298b17c44ff22d922516028da31cec14d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -623,6 +623,7 @@ is_uncached_acl(struct posix_acl *acl)
 #define IOP_NOFOLLOW	0x0004
 #define IOP_XATTR	0x0008
 #define IOP_DEFAULT_READLINK	0x0010
+#define IOP_MGTIME	0x0020
 
 /*
  * Keep mostly read-only and often accessed (especially for
@@ -2581,7 +2582,7 @@ struct file_system_type {
  */
 static inline bool is_mgtime(const struct inode *inode)
 {
-	return inode->i_sb->s_type->fs_flags & FS_MGTIME;
+	return inode->i_opflags & IOP_MGTIME;
 }
 
 extern struct dentry *mount_bdev(struct file_system_type *fs_type,

---
base-commit: 80ce1b3dc72ceab16a967e2aa222c5cc06ad6042
change-id: 20241113-mgtime-9aad7b90c64a

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>